Instead of serving three years, Michael Wheeler, 36, will now have to spend four and a half years behind bars after the Court of Appeal agreed with Lord Goldsmith, the Attorney General, that the original sentence had been unduly lenient.

Increasing the sentence, Lord Justice Kay issued a warning that men who use the internet to prey on young girls risked even more substantial prison terms.

However, the mother of one of the 13-year-old girls whom Wheeler had seduced after first contacting her in a chatroom when she was 11, said she was disgusted he had not been given at least 10 years in prison.

Wheeler, an electronics engineer from Cambridge, posed as a 16-year-old when he went into chatrooms and had sex with two girls shortly after their 13th birthdays. Police believe he waited until then because the maximum sentence for unlawful sexual intercourse with a girl 12 or under is life.

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The maximum sentence for sex with a girl aged 13-plus is currently two years, though there are legislative proposals to increase it to 14 years.

Wheeler was sentenced to three years at Norwich Crown Court earlier this year after admitting three counts of unlawful sex with the girl and three of indecently assaulting her. He also admitted two counts of unlawful sex with the girl's friend, 13, and three further counts of indecent assault.

On Friday, Lord Goldsmith asked three appeal judges to increase the sentence. David Perry, for the Attorney General, said: "The essential submission on behalf of the Attorney General is that three years failed adequately to reflect the gravity of the offences, in particular the need for deterrence and for public concern about offences of this nature.

"The use of the internet by older men to contact young girls and then prey upon young and vulnerable girls is a matter of profound public concern." Increasing both the prison term and the amount of time Wheeler will spend on licence - by six months to two and a half years - Lord Justice Kay commented: "Men contemplating involvement with much younger girls via internet chatrooms with a view to sexual activity should appreciate that the court is likely to pass a longer sentence than this in future.

"When Parliament has finished its reconsideration of the law on sexual offending, the courts may be able to take an even more punitive view of such offences."

Families of the girls had hoped that Wheeler would get an even longer sentence.

"When I heard about the new sentence, I thought it was disgusting," said one mother. "I had 10 years in my mind. It still doesn't seem very long until he will be let out of prison.

"My daughter is worried about what will happen when he gets out. One of the worst things is the fact that he doesn't seem to regret what he's done.

The mother of the second girl said Wheeler's crimes had affected her family so badly that she had been off work for 13 weeks and her daughter was now suffering from depression.

Det Insp Neil Smith, who led the investigation, said that Wheeler posed a serious danger to young girls. "He knew how vulnerable these girls would be."